2015/12/19

SICP 問題 4.06

letを導入．
lambdaに変形することで定義する．

;; (let ((a 1) (b 2) (c 3))
;;   (+ a b c))

;; ((lambda (a b c)
;;    (+ a b c)) 1 2 3)

(define (let? exp) (tagged-list? exp 'let))
(define (let-parameters exp) (cadr exp))
(define (let-variables exp) (map car (let-parameters exp)))
(define (let-expressions exp) (map cadr (let-parameters exp)))
(define (let-bodys exp) (cddr exp))

(define (let->lambda exp)
(cons (make-lambda (let-variables exp)
(let-bodys exp))
(let-expressions exp)))
gosh> (let->lambda '(let ((var1 exp1)
(var2 exp2)
(var3 exp3))
body1
body2))
((lambda (var1 var2 var3)
body1
body2)
exp1 exp2 exp3)
(define (eval exp env)
(cond ((self-evaluating? exp) exp)
((variable? exp) (lookup-valiable-value exp env))
((quoted? exp) (text-of-quotation exp))
((assignment? exp) (eval-assignment exp env))
((definition? exp) (eval-definition exp env))
((if? exp) (eval-if exp env))
((lambda? exp) (make-procedure (lambda-parameters exp)
(lambda-body exp)
env))
((let? exp) (eval (let->lambda exp) env)) ;;letを追加
((begin? exp)
(eval-sequence (begin-actions exp) env))
((cond? exp) (eval (cond->if exp) env))
((and? exp) (eval (and->if exp) env))
((or? exp) (eval (or->if exp) env))
((application? exp)
(apply (eval (operator exp) env)
(list-of-values (operands exp) env)))
(else
(error "Unknown expression type: EVAL" exp))))